The subject invention relates to improvements in the measurement of magnetic fields in terms of providing a new laboratory instrument and also in terms of providing a new magnetometer for antisubmarine warefare (ASW) applications in which individual magnetometers are carried by air-dropped or moored water-born buoys, which are distributed over a given area to monitor either subsurface or surface activity.
It is well known that submarines and surface ships alter the local magnetic fields in the vicinity of the ship. In prior art ASW surveillance systems, a so-called "point" magnetometer is carrie don a boom mounted aft of an aircraft and the aircraft executes a search pattern in search of changes of the local magnetic field which would indicate the presence of a submarine or a surface ship. It will be appreciated that in this mode of operation only one point immediately below the point magnetometer is searched at a time and that a submarine or surface ship may avoid detection simply by not being at the point searched at the time that the magnetometer is overflying the point.
Up to the present time, it has been inexpedient to provide water-born buoys with magnetometers due to a number of factors. The first factor which is almost determinative of the situation is the power drain when utilizing present day "point" magnetometers. The usual point magnetometer is the so called nuclear magnetic resonance (NMR) spectrometer or optically pumped magnetometer which involves the use of a continuous pumping beam and the use of an RF feedback for stimulating the natural precession of the magnetic moments or magnetic angular momentum vectors of atoms in a gas cell about a local magnetic field. The use of a continuous pumping beam of sufficient magnitude to stimulate the gas would run down currently available batteries utilized in water-born buoys in a matter of minutes, thereby precluding the use of this type of system in buoys whose life must be in excess of one hour.
Moreover, in nuclear magnetic resonance spectrometers there is a problem with the intensity stability of the source. It is well known that the presence of a continuous pumping beam actually shifts the atomic precession frequency away from the value which it would have had in the absence of this beam. Fluctuations in pumping light intensity translate into fluctuations in atomic precession frequency, which fluctuations are indistinguishable from those caused by changes in magnetic field strength. Great sensitivity in such a magnetometer therefore requires an extremely stable light source, and the construction of a light source of this type is quite costly. Moreover, the maintenance of a stable light source in a buoy environment is all but impossible.
An additional power draining feature in prior art systems is the feature which requires the generation of a periodic magnetic field in the RF region of the electromagnetic spectrum to stimulate the Larmor precession. Not only does this additional source draw energy from the battery continuously, but the RF field also affects the measurement in the same manner as the continuous pumping beam by engendering shifts in the atomic resonance frequency (Bloch-Siegert Shift). It will be appreciated that most previous optically pumped magnetometers have required the presence of the RF magnetic field to drive the atomic precession and thus render it susceptible to observation.
Moreover, the alignment of the RF field with the pumping beam axis is extremely critical. It will be appreciated that the maintenance of this alignment in a buoy application is difficult due to the shock experienced when the buoy hits the water.
While the above problems exist specifically with respect to the water-born buoy application mentioned above, laboratory instruments involving the use of NMR spectrometers also suffer from the effect of the continuous pumping beam on frequency and the effect of the RF field on the measurement being made.
The above problems are solved by the subject invention in which, in one embodiment, a circularly polarized "pulse" of light from a high intensity source such as a laser is projected through a gas cell, with the wavelength of the light corresponding to a predetermined transition of the gas. A relatively weak continuous probe beam at this transition is projected through the gas cell in a direction normal to the direction of propagation of the "pulse". After the passage of the pulse through the cell, due to the high intensity of the pulse the angular moments of large numbers of atoms line up in the direction of the propagation of the "pulse" and thereafter freely precess about the local magnetic field at the Larmor frequency. The free precession frequency, which is proportional to the local magnetic field, is then read out in terms of the frequency of the amplitude modulation of the probe beam caused by the rapid periodic population shifts between weakly and strongly absorbing magnetic sublevels of the atoms in the gas as the atoms precess.
The purpose of the pumping source is to pump up large numbers of atoms in the gas cell. Because of the high degree of polarization which can be obtained by a single pulse of the laser, free precession can be observed for a relatively long period of time before the signal has decayed to an unobserved level. Thus, no reinforcement of the free precession is necessary.
Once pumping is accomplished, pumping energy is no longer needed and is, in fact, undesirable. By using a pulse of pumping energy, instead of continuous illumination, the effect of the pumping beam on the measurement is eliminated because once the pulse exits the cell, it has no effect on the gas in the cell.
While it is true that the presence of the probe beam will tend to shift the atomic precession frequency, this beam is much less intense than the beam used for pumping and its effect is correspondingly reduced and made quite negligible. It will be appreciated that the gas utilized is desirably one which has a zero orbital angular momentum state (S state), containing one or several unpaired electrons. Alternatively, other states having unpaired electrons may be utilized.
The S state is desirable because of its zero orbital angular momentum, which relaxes or decays at a slower rate than do states having non-zero orbital angular momenta. It is a finding of this invention that the decay is sufficiently slow to permit observation of the free precession which is measured by the subject system.
The magnitude of the magnetic field at the gas cell is determined from the frequency of the amplitude modulation of the probe beam. The probe beam is modulated at the frequency, .omega., of the precessing atoms in the gas by a process involving population shifts between weakly and strongly absorbing angular momentum sublevels after pumping has occurred. As is generally understood, the periodic population shift takes place at the same frequency as that of the precession. In this case, "free precession" refers to the absence of any external reinforcement of the natural precession either by feedback loops or otherwise. It is a finding of this invention that when using a high energy pumping pulse no feedback or direct stimulus is necessary in order to read out the Larmor frequency of the freely precessing atoms. While absolute value of the magnetic field may be obtained from the subject apparatus, in ASW applications, changes in the magnetic field, dHdt, representative of a passing ship or submarine may be detected without the detection of the absolute value of the magnetic field.
As will be appreciated, the obtaining of free precession without RF feedback eliminates the need for a direct coupled alternating magnetic field, the power associated with the generation of the field and the frequency shift engendered by the use of such a field, as well as the problem of aligning the RF field coils.
Secondly, by the use of a pulsed source, the effect of the source on the measurement is completely eliminated as well as reducing the power requirements by eliminating the need for a continuous beam. The source is pulsed at a rate which, in general, will be much longer than the rate of Larmor precession for the particular gas and the expected magnetic field. In water-born buoy operation to conserve power the source may be pulsed at a rate as low as 5 times a second. Finally, the requirement for a stable source is eliminated because the pumping beam has no effect on the observed resonance frequency.
The subject system has unique application to water-born buoys outfitted with the subject magnetometer because of the low power drain and the relatively limited effect of the marine environment on this type of magnetometer. It will be appreciated that for ASW applications it is only the change in magnetic field due to the passage of a ship or submarine, as opposed to the absolute value of the magnetic field, which need be monitored. The motion of the buoy due to wave action or currents can be designed to be outside of the change in magnetic field value expected due to the passage of surface or surface vessels, even though the motion of the buoy would have an effect on measurement of the absolute value of earth's magnetic field at the buoy.
As a laboratory instrument, the subject magnetometer is easily controlled with stability and alignment being considerably less critical than in the prior art optically pumped magnetometers. Moreover, the pumping source may be a low power semiconductor laser or a multimode laser which emits a relatively broad spectrum of radiation, one peak of which will be at the appropriate transition of the atoms in the gas.
While most optically pumped magnetometers have utilized RF feedback at the Larmor frequency, as illustrated in U.S. Pat. No. 3,173,082 issued to W. E. Bell et al on Mar. 9, 1965, one embodiment of a magnetometer is described in the Bell et al patent in which a directly-coupled alternating magnetic field is not used. In Bell et al, embodiment of FIG. 1 a continuous source is modulated at .omega.=Larmor precession frequency to "reinforce" the natural precession and the field is read out as the absolute amplitude of the pumping beam rather than .omega..
In this embodiment of the Bell et al system, there is no free precession because the modulation of the pumping beam "reinforces" the precession. Moreover, there is no pulsed source, as in the subject system. The Bell et al source is continuous, and interaction with the gas is required, while in the subject system, care is taken to prevent interaction of the pumping beam and the gas once the precession phenomenon has been initiated as a result of the pumping.
Moreover, Bell et al do not use a separate probe beam and do not measure "modulation" of this probe beam. Bell et al do measure the modulation of the pumping beam (see FIGS. 5-7). However, in the embodiments of FIGS. 5-7 regenerative feedback is employed.
Thus, unlike the Bell et al system, the subject system is pulse pumped and does not depend on reinforced or stimulated precession in order to detect the atomic resonance precession frequency.
It is therefore an object of this invention to provide an improved magnetometer utilizing pulse pumping.
It is a further object of this invention to provide a magnetometer in which free precession is allowed to exist and from which a measurement of the local magnetic field is made.
It is another object of this invention to provide a magnetometer suitable for use in ASW applications either from moored buoys or from expendable airdropped buoys in which power drain and stability requirements are minimized.
It is another object of this invention to provide an improved ASW system utilizing a pulsed pumped magnetometer.
It is still further object of this invention to provide an improved ASW system utilizing a magnetometer operating on the principle of free precession.
It is a still further object of this invention to provide a low cost, easily fabricated magnetometer.
These and other objects will be better understood in conjunction with the following specification taken in conjunction with the drawings in which: